The present invention relates to a transceiver using a single carrier frequency for full-duplex transmission.
Two different frequencies are generally required for transceivers to carry out communication simultaneously in both directions. However, a full-duplex transmission using a single carrier frequency so as to efficiently use radio frequencies has been proposed in the paper titled "Simultaneous Transmitting and Receiving Method Mobil Radio Using Only One Frequency Channel", Proceedings of the 1992 IEICE Spring Conference, Paper No. B-769, page 3-336, Mar. 15, 1992.
This method will be described in detail with reference to FIGS. 3 and 4. A voice signal supplied from a voice input unit 1 such as a microphone is amplified and band-limited by an audio amplifier 2. This voice signal is then supplied to a time compressor 3 to undergo signal processing such as a process of compressing time two times or more and a process of assigning a synchronizing signal outputted from a synchronizing signal generator 19 for giving transmission/reception timings. With this transmission/reception timing process, at least the first transceiver sending the voice signal transmits the synchronizing signal in order for the second transceiver receiving the signal from the first transceiver to generate its transmission/reception timings. The time compression approximately doubles the frequency band of the original voice signal. The time compressed voice signal is frequency modulated by a frequency synthesizer. The modulated signal is mixed with a signal supplied from an oscillator 6 by a frequency converter 5 so as to generate a signal having a transmission frequency. This signal is amplified to a sufficient power by a power amplifier 7 and supplied via an antenna switch 8 to an antenna 9 from which it is radiated.
In receiving a signal from the second transceiver, a signal received by the antenna 9 is supplied via the antenna switch 8 to a radio frequency amplifier 10. The amplified reception signal is mixed with a signal supplied from the frequency synthesizer 4 so as to be converted into an intermediate frequency signal which is detected or demodulated by a demodulator 11. Since there is no voice input from the voice input unit 1 during the reception period, the frequency synthesizer 4 does not modulate a voice signal.
The synchronizing signal contained in the detected signal is picked up by a synchronizing signal detector 12 synchronously with transmission/reception timings, and supplied to a controller 13. Synchronously with this synchronizing signal, the controller 13 generates a timing Control signal and supplies it to the time compressor 3, antenna switch 8, oscillator 6, time expander 14, and synchronizing signal generator 19.
In the detected signal, the voice signal having a doubled frequency band passes through the synchronizing signal detector 12 and is expanded to the original signal by a time expander 14 under the control of the controller 13. This signal is then amplified by an audio amplifier 15 and supplied to a voice output unit 16 such as a loudspeaker.
FIG. 4 is a timing chart explaining the operation of the circuit shown in FIG. 3. A voice input signal indicated at (a) undergoes a time compression process and is converted to a time compressed signal indicated at (b). Reference numeral 18 represents a synchronizing signal. This time compressed signal is frequency modulated by a signal generated by the frequency synthesizer 4, and as shown at (d), transmitted at a transmission timing (R representing a reception time period, and T representing a transmission time period) indicated at (c) from the antenna 9. The voice signal indicated at (a) by oblique lines is time compressed and transmitted at the transmission timing as shown at (d). During the reception time period, a signal which is time compressed, modulated, and transmitted by the second transceiver is received in the first transceiver.
As shown at (d) in FIG. 4, the first transceiver transmits a synchronizing signal 18 at the transmission timing, and the transmission/reception timings of the second transceiver are controlled synchronously with the synchronizing signal 18.
With this conventional method, however, even if there is no input signal, a carrier signal is transmitted at a constant time interval synchronously with the synchronizing signal 18 while transmissions and receptions are alternately performed at the constant time interval. There arises therefore a problem of wasteful radio waves and inefficient use of them.
Furthermore, because two transceivers are always transmitting frequency modulated radio waves alternately, even if a third party which can radiate only a weak electromagnetic field tries to intrude into communication between the two transceivers, the third party cannot intrude into it. Still further, because a power consumption of a transceiver is larger during transmission than during reception, the transmission power consumed while there is no voice signal becomes more wasteful.